One type of color image-reading device, (i.e., scanner) includes an optical unit which includes a light source, a photoelectric conversion device, and optical elements such as reflective mirrors and lenses that form a light path guiding light reflected from a document to the photoelectric conversion device, that is moved along the document in order to obtain image data from the document. The optical unit is mounted on a carriage that is moved by a driving mechanism that includes a driving belt, such as a timing belt, that is moved back and forth by the output rotation of a driving motor. The carriage is connected to this driving belt to move back and forth with it. The carriage is housed in a casing. In order to guide the movement of the carriage, a guide rod extending in what is termed the length direction of the casing is connected to the carriage.
FIG. 8 shows an exploded perspective view of the basic structure of an image-reading device 1 that is a conventional example of this type of image-reading device. The image-reading device 1 includes a carriage 5 that is supported on a casing that includes a top base plate 3a and a bottom base plate 3b so that the carriage is free to move in the length direction of the casing. The carriage 5 includes an optical unit 7 that includes a light source, a photoelectric conversion device and optical elements. Movement of the carriage 5 is guided by guide rods 9a and 9b that are on opposite sides of the casing in what is termed the width direction, that is, a direction perpendicular to the length direction, which is the direction of elongation of the carriage when it is mounted in the casing. The guide rods 9a and 9b pass through guide holes in guided parts 5a and 5b provided on the side of the carriage 5. The ends of the guide rods 9a and 9b extend in the length direction and are anchored by screws 9c to brackets 11a and 11b attached to the ends of a bottom plate 3b. On the outside of the guide rod 9b, a drive belt 13 is provided along the guide rod 9b, and the drive belt 13 is anchored to the guided part 5b. This drive belt 13 is looped over a drive-side pulley 15a that is supported on the bracket 11a so as to be free to rotate, and a driven-side pulley 15b that is supported on the bracket 11b so as to be free to rotate. A drive motor 17 and a circuit board 19, which includes the drive circuits for the drive motor 17 and control circuits for the photoelectric conversion device of the optical unit 7 are mounted on the bracket 11a. In addition, appropriate gears 21a and 21b that convert the output rotations of the drive motor 17 to transmit such to the drive-side pulley 15a are supported on the bracket 11a. The optical unit 7 and the circuit board 19 are connected by a flexible circuit board 23.
The top base plate 3a is formed in the shape of a rectangular frame, and a glass plate 25 is mounted inside this frame. A document to be scanned is placed on the glass plate 25. The bottom base plate 3b is formed in a rectangular shape and is attached to the top base plate 3a by screws 3c in order to form a casing that houses the various components of the image-reading device 1.
Both the top base plate 3a and the bottom base plate 3b of the casing are formed by injection molding a material such as plastic (i.e., synthetic resin). However, the image-reading device 1 needs to be of sufficient size to enable the reading of documents recorded on, for example, A4 size paper. Accordingly, in molding the casing, it is necessary to have a metal mold that can mold products of a size sufficient to scan such documents. Additionally, parts of the frame-shaped top base plate 3a and the rectangular-shaped bottom base plate 3b are of complex shapes, and therefore the metal mold structure must be complex. This causes the production costs to increase.